It is the stuff dreams are made of. On a whim, John Maloof spent $400 on a trunk full of negatives at a local auction house. It turned out to be the priceless archive of an amateur street photographer named Vivian Maier.

Maloof wasn't a photography buff; he was researching a book about Chicago history. But he soon became one, establishing an attic darkroom to develop some of the 100,000 negatives he had managed to acquire.

Negatives from the razing of the Federal Building, 1965, by Vivian Maier. Photo via ChicagoGeek/Flickr.

The results were stunning. Maier was a brilliant photographer of urban scenes and a talented portraitist, and her archive is an unparalleled record of mid-century Chicago. So why were her negatives stashed rudely in a trunk at the auction house instead of developed and shared with the world?

That's what Maloof sets out to learn in Finding Vivian Maier, a documentary whose trailer was released in February: